1. Field of the Invention
The invention relates generally to a scanning micromirror used in optical communication systems, and more particularly to, a 2-axis tilt scanning micromirror having vertical comb-drive actuators, and method of manufacturing the same.
2. Description of the Prior Art
Micro-electro-mechanical system (MEMS) is a technology of miniaturization and integration of mechanical and electronic devices. With this technology one can precisely manufacture very small electro-mechanical devices on a semiconductor wafer using processes such as lithography, deposition, etching, or the like and integrate multi-functional devices on one chip.
Recently, in the optical communication systems, there is explosive demand on bandwidth due to network traffic increase. MEMS can be a key enabling technology in this optical network. It is expected that current optical cross-connect (OXC) system including optical-electronic-optical (O-E-O) conversion will confront a bottleneck phenomenon due to rapid increase of the data transmission rate. MEMS technology is thought to be effectively used to overcome this obstacle and move oil to an all-optical system without necessity of O-E-O conversion by providing an array of scanning micromirrors.
OXC using MEMS technology can be classified into 2-D and 3-D type depending on the system configuration. 2-D OXC employs digitally-controlled optical matrix switches and 3-D OXC employs an analog scanning micromirror array. As 2-D OXC is driven digitally it has advantages of easy control, high reliability, and relatively easy optical alignment. However, in case of a Nxc3x97N OXC, it requires N2 micromirrors and its optical insertion loss is significantly increased as the number of the input/output port is increased. Therefore, experts in the art expect that the maximum number of the port will be limited to 32xc3x9732.
On the other hand, 3-D OXC requires additional complicated control scheme such as closed-loop feedback control, etc., since it uses analog micromirrors and optical alignment in this case is very difficult. However, it only requires 2N micromirrors in case of Nxc3x97N OXC. Further, it is very advantageous in terms of insertion loss when that the number of the input/output port is over 32 owing to its high scalability.
In 3-D OXC, the scanning micromirror is rotated by actuators so that an optical signal is routed from a single input optical fiber to a single output optical fiber. In other words, it serves to change a path of the optical signal. Generally, the scanning micromirror employs electrostatic, electromagnetic, piezoelectric, thermal actuation mechanism, and the like. Among them, electrostatic actuation has low power consumption, relatively high speed and matured fabrication technology. It is thus considered as the most suitable actuation mechanism for 3-D OXC. In addition, this scanning micromirror is used for an optical add-drop multiplexer (OADM), a wavelength selective cross-connect (WSXC), or the like, in the optical network, as well as a high-resolution display, an optical scanner, or the like.
Currently, the scanning micromirror with the electrostatic actuators for 3-D OXC employs a parallel-plate type actuator of which design is relatively easy. For example, U.S. Pat. No. 5,914,801 (hereinafter called xe2x80x98reference document 1xe2x80x99) entitled xe2x80x9cMicroelectro-mechanical Devices Including Rotating Plates and Related Methodsxe2x80x9d by xe2x80x98V, R. Dhuler, et al.xe2x80x99 (Jul. 22, 1999) discloses a microelectromechanical device, which is driven by the parallel-plate type electrostatic actuator and is rotated about two axes. The reference document 1 discloses a technical idea that the scanning micromirror includes a first frame, a second frame and a plate, each of which is connected through beams, and the plate is rotated about two axes by the parallel-plate type electrostatic actuator to change the optical path.
The scanning micromirror employing the parallel-plate type electrostatic actuators, however, has disadvantages that whole range of rotation angle cannot be used due to a pull-in phenomenon, and the response speed is low due to squeezed air damping.
The present invention is contrived to solve the above problems and an object of the present invention is to provide a scanning micromirror for optical communication having a large rotation angle and rapid response speed due to low air damping.
Another object of the present invention is to provide a method of manufacturing a scanning micromirror for optical communications having a large rotation angle and rapid response speed due to low air damping.
In order to accomplish the above object, the scanning micromirror for optical communications according to the present invention, is characterized in that it comprises an outer frame having an aperture therein, an inner frame located within the aperture of the outer frame and having an aperture therein, an optical reflecting means located within the aperture of the inner frame, a plurality of first torsion springs connected between an inner wall of the outer frame and an outer wall of the inner frame, for supporting the inner frame, a plurality of second torsion springs connected between the inner wall of the inner frame and the optical reflecting means, for supporting the optical reflecting means, a pair of first comb-type electrostatic actuators electrically isolated from each other for applying electrostatic torques by which the inner frame is rotated bi-directionally about an axis of the first torsion springs, and a pair of second comb-type electrostatic actuators electrically isolated from each other for applying electrostatic torques by which the optical reflecting means is rotated bi-directionally about an axis of the second torsion springs.
At this time, it is preferred that the first comb-type electrostatic actuators include first fixed combs of which one ends are connected to the inner wall of the outer frame and the other ends are free, and first movable combs of which one ends are connected to the outer wall of the inner frame and the other ends are free, wherein the first movable combs are interdigitated with the first fixed combs.
Also, it is preferred that the second comb-type electrostatic actuators include second fixed combs of which one ends are connected to the inner wall of the inner frame and the other ends are free, and second movable combs of which one ends are connected to the optical reflecting means and the other ends are free, wherein the second movable combs are interdigitated with the second fixed combs.
In order to accomplish another object, the method of manufacturing the scanning micromirror for optical communications according to the present invention, is characterized in that it comprises the steps of (a) forming a first upper silicon dioxide layer and a lower silicon dioxide layer on both sides of a silicon-on-insulator (SOI) wafer comprising a handling silicon layer, a buried silicon dioxide layer and a silicon device layer, (b) depositing a polysilicon layer on the first upper silicon dioxide layer and then patterning the polysilicon layer using a mask for defining a portion of the first fixed combs, the second fixed combs, the first torsion springs, the outer frame and the inner frame, to which the second fixed combs are connected, (c) depositing a second upper silicon dioxide layer on the entire structure and then patterning the: second upper silicon dioxide layer and the first upper silicon dioxide layer using a mask that defines a portion of the remaining polysilicon layer, the first movable combs, the second movable combs, the optical reflecting means, the second torsion springs and the inner frame, to which the first movable combs are connected, (d) patterning the lower silicon dioxide layer using a mask that defines a portion in which the handling layer is thinned down to have a given thickness, (e) etching the silicon device layer using the first upper silicon dioxide layer as a mask, (f) removing the second upper silicon dioxide layer, etching the first upper silicon dioxide layer using the polysilicon layer as a mask and etching the buried silicon dioxide layer using the silicon device layer as a mask, (g) etching the handling silicon layer so that it has a given thickness, using the lower silicon dioxide layer as a mask, (h) removing the polysilicon layer, etching the silicon device layer using the first upper silicon dioxide layer remaining below the polysilicon layer as a mask and then etching the handling silicon layer using the buried silicon dioxide layer as a mask, and (i) removing the first upper silicon dioxide layer and the lower silicon dioxide layer, removing the buried silicon dioxide layer using the remaining silicon device layer as a mask and then depositing a metal layer on upper surface and lower surface of the entire structure.